Power amplifiers used in transmitters may be optimized for use in a particular mode and frequency band to maximize efficiency. Such optimization may require the amplifier to be biased in a certain manner. Additionally, impedances usually need to be matched between components within the amplifier and between the amplifier and adjacent components.
Difficulties arise, however, with the demands some communication systems place upon an amplifier. For example, in a W-CDMA or CDMA2000 transmitter, a signal with a non-constant envelope is traditionally fed through a power amplifier. However, it is difficult to reach optimum levels of amplifier efficiency and linearity: design compromises between the two are often required. Moreover, a wide range of output power is required: typically on the order of 80 dB.
Further difficulties may arise with multiband transmitters. For example, impedance is dependant on the operating frequency, and therefore, an amplifier having optimal impedance matching in one frequency band will not be optimized for operation in a different frequency band. Problems related to impedance matching at different frequencies may be solved by providing separate amplifying chains. Separate amplifying chains, however, can be costly, increase the size of the transmitter and increase the power required for the transmitter.
Amplifier design and impedance matching is further made difficult because, in present communication systems, it is desirable for an amplifier to operate over multiple frequency bands. For example, a transmitter may be used in GSM900 (880–915 MHz) and DCS1800 (1710–1785 MHz bands). As another example, a transmitter may be used in CDMA800 (824–849 MHz) and CDMA1900 (1850–1910 MHz) frequency bands. Typically, dual-band mobile phone transceivers contain two power amplifiers, each operating within a single frequency bandwidth, and each requiring impedance matching.
The prior art has attempted to provide solutions to amplifier design and impedance matching difficulties. For example, FIG. 1 shows one prior art attempt at impedance matching for a dual-band single-stage power amplifier operating in either the 800 MHz or the 1900 MHz bands. A single active device with switching impedance networks 104 and 106 at the input, amplifier 102, bias control 103, voltage source 107 switching impedance network 105 and switching impedance networks 108 and 110 at the output, to provide desired input and output impedances. The need for these switching impedance networks, however, drives up the cost of the device and drives down the efficiency.
Another approach to impedance matching in a dual-band power amplifier is shown in FIG. 2. Amplifier 214 is matched to a first matching circuit 202. A second matching circuit 204 consisting of two separate impedance networks 206 and 208 is tuned to each frequency bandwidth. Two switches 210 and 212 are necessary to this approach. This approach again drives up the cost of the device as well as driving down efficiency.
Another prior art approach is to ignore efficiency considerations. For example, low-efficiency Class A operation matching circuits for multiband, single-stage power amplifiers may be used. These are based on field-effect transistors (FETs). In these circuits, the difference between power gain for 800 MHz and 1900 MHz is very significant, typically about 15 dB. However, if a consistent output power is desired, such as in 2.5 G and 3 G communication systems, different input power needs to be applied to the different amplifiers, which may create further design difficulties.
Yet another prior art approach is to provide multistage power amplifiers. Multistage power amplifiers may be generally desirable as they may provide increased input resistance, increased gain and increased power handling capability, when compared to single stage power amplifiers. However, implementations to date, such as shown in multi stage embodiments of the device seen in FIG. 1, have power drain as well as device cost difficulties. For example, the prior art embodiment shown in FIG. 1 may be utilized in a multistage power amplifier. However, the number of components such as impedance networks and switches is increased in this type of approach, thus increasing cost, size and inefficiency of the system.
Accordingly, there is a need for a low cost, high efficiency multiband amplifier capable of providing appropriate output power across multiple frequency bands.